In the fabrication of a semiconductor device, electrical contacts are typically made by opening contact holes in a dielectric isolation layer to expose regions of the semiconductor device to which electrical contact must be made, and depositing a layer of metal over each contact hole. The layer of metal extends from the bottom of the hole, up a side wall of the hole and over the dielectric isolation layer to another contact hole or to a contact pad overlying the dielectric isolation layer.
The layer of metal, typically Al-Si, is deposited by DC magnetron evaporation or electron beam evaporation. Such physical deposition techniques provide good coverage for horizontal surfaces, but do not always provide good coverage for vertical surfaces. Thus, the metal layer may be unacceptably thin or interrupted at the side walls of contact holes when these deposition techniques are used, and the required electrical contacts may be unreliable or defective.
Metal coverage at the side walls may be improved by sloping the side walls, either by etching the dielectric isolation layer adjacent to the contact holes or, where the dielectric isolation layer is formed from a glass, by heating the glass to cause it to reflow. However, sloping side walls increase the surface area required for the contact holes, thereby increasing the total area required for the device. Moreover, heating the device to a temperature sufficient for glass reflow causes unwanted diffusion of dopants within the semiconductor device.
Alternatively, the problem of metal coverage at the side walls may be avoided by filling the contact holes with metallic plugs and depositing a metal such as Al-Si over the dielectric isolation layer and metallic plugs to define interconnections and contact pads.
The metallic plugs may be formed by employing chemical vapour deposition (CVD) to deposit a thick conformal layer of a refractory metal such as tungsten over the entire device to a thickness sufficient to fill the contact holes, and etching back the tungsten to expose the dielectric isolation layer adjacent the contact holes while leaving tungsten plugs in the contact holes. Unfortunately, the conformal CVD and etch back processes are relatively slow, and the deposited tungsten layer must be sufficiently thick to fill the largest contact holes. Consequently, this method of filling contact holes is time consuming, particularly where relatively large contact holes are required. Moreover, the tungsten does not bond well to the side walls of the contact holes and may heave during subsequent process steps, making the contact unreliable or defective.
In an alternative method of filling contact holes, selective CVD may be employed to deposit a refractory metal such as tungsten only on exposed silicon at the bottoms of the contact holes. The selective CVD may be continued until the tungsten fills the contact holes. Because the tungsten is selectively deposited in the contact holes, etching of unwanted tungsten is required only when one or more of the contact holes is overfilled. However, this alternative method is also time consuming since the tungsten is selectively deposited only at the bottom of the contact holes and not at the side walls. Moreover, the tungsten does not bond well to the side walls of the contact holes as in the previous alternative method.